We have recently coupled microdialysis with coaxial continuous flow FAB mass spectrometry. This technique allows for the possibility of determination in real time of many biochemicals in the body. Microdialysis is a relatively new sampling technique designed for the in vivo analysis of chemical substances. The microdialysis probe is surgically implanted into the tissue or area of interest in the animal . This in vivo technique is advantageous since there is minimal damage to the sampling site and it does not alter the fluid balance. The present investigation involves the possibility of determining the half-life of a tris organophosphate, tris (2-chloroethyl) phosphate (TRCP), in rats. Several of these tris organophosphates have been widely used in plastics and synthetic fibers as flame retardants and are suspected carcinogens. In our initial experiments, a decline in the amount of TRCP in the blood stream of anesthetized rats was observed over a three hour time period after dosing the animal. We have just completed a surgical protocol in order to perform the same experiments in awake, freely-moving animals. This will allow a more realistic determination of the physiological levels in the animal. In the future, we will be performing microdialysis in the brain of rats in an effort to determine the levels of endogenous neuropeptides and neurotransmitters in real time.